Method for producing acrylonitrile

ABSTRACT

The present invention relates to a method for producing acrylonitrile comprising the steps of reacting propylene and/or propane, ammonia and molecular oxygen in the presence of a catalyst to form a reacted gas, contacting the reacted gas with a liquid containing water and an acid in a multi-stage quenching tower having three or more chambers and transferring the liquid contacted with the reacted gas in the second quenching chamber to the first quenching chamber. According to the present invention, high boiling point compounds are efficiently removed and clogging is prevented in an acrylonitrile recovery process so that the productivity of acrylonitrile is enhanced.

This application is a 371 of PCT/JP96/00402 filed Feb. 22, 1996,WO96/26917, Sep. 6, 1996.

TECHNICAL FIELD

The present invention relates to a method for producing acrylonitrile byammoxydation, more particularly to a method for facilitating recovery ofacrylonitrile by efficiently removing high boiling point compounds(heavies) and the like in the process of quenching a reacted gas.

BACKGROUND ART

The method for producing acrylonitrile by ammoxydation has been improvedin various ways since its success in industrialization, and it is saidthat the method has technically matured. Still, efforts have been madeto improve unit consumption of raw materials and utility, and to reduceproduction cost by achieving production efficiency.

It is well known that reacted gas obtained by reacting raw materialscontains by-products such as hydrocyanic acid, acetonitrile and aldehydein addition to acrylonitrile when acrylonitrile is produced byammoxydation. The by-products react with acrylonitrile in the presenceof unreacted ammonia or react with one another; as a result, highboiling point compounds are produced. The high boiling point compoundscause not only reduction in the yield of acrylonitrile, the targetproduct, but also clogging at various places in towers in the subsequentprocesses. Consequently, the by-products must be immediately separatedfrom the reacted gas.

Conventionally, a method has been adopted for separating unreactedammonia in the form of a salt and simultaneously removing otherimpurities and the by-products mentioned above by preliminarily coolingthe reacted gas produced in a reactor, then transferring the cooledreacted gas to a quenching tower immediately, and washing and quenchingthe reacted gas with water containing acids such as sulfuric acid. Asone of the conventional methods, there is proposed a method employing amulti-stage quenching tower to divide a quenching process into two stepsor more, which comprises separating most of the unreacted ammonia in theform of a salt by contacting the water containing a sufficient amount ofsulfuric acid to neutralize the unreacted ammonia and simultaneouslycondensing a part of the water vapor contained in the reacted gas in thefirst chamber, and condensing most of the residual water vapor in thesecond chamber (U.S. Pat. No. 3,649,179).

This method has succeeded in reducing the cost for separating andtreating the salt because of its higher concentration of the resultantammonium salt than that in the conventional methods. Yet, the gasintroduced from the first quenching chamber to the second quenchingchamber contains unreacted ammonia, high boiling point compounds,polymers and scattered catalysts which are not removed in the firstquenching chamber. A liquid containing high boiling point compounds,polymers, scattered catalysts and an ammonium salt is transferred in theform of mist together with the gas. Therefore, the liquid dischargedfrom the second quenching chamber contains these impurities. To recoveracrylonitrile contained in the discharged liquid, impurities should beremoved.

Heretofore, the impurities contained in the discharged liquid wasremoved in an acrylonitrile recovery process or a waste water column;however, the impurities clogged a recovery column, a stripper andreboilers and pipes when the discharged liquid was transferred to theacrylonitrile recovery process. A large quantity of steam is required totransfer the discharged liquid to a waste water column for recovery ofacrylonitrile from the discharged liquid. Therefore, the method was noteconomical.

Further, in the method of the above U.S. patent, if the liquiddischarged from the second quenching chamber is sent back to the firstquenching chamber in order to decrease the content of high boiling pointcompounds and the like in the discharged liquid, the same effect will beexhibited as in the case that a gas is treated in a one-stage quenchingtower. In other words, the effect of the two-stage quenching towercannot be obtained. Accordingly, the problems such as acrylonitrile lossand clogging due to high boiling point compounds have still remained.

DISCLOSURE OF THE INVENTION

The present inventors have made extensive and intensive research tosolve the above problems and found that the high boiling point compoundsare efficiently removed and that the clogging is effectively preventedby improving the treatment method of the liquid discharged from aquenching tower. As a result, they achieved the present invention.

This invention provides a method for producing acrylonitrile comprisingthe steps of reacting propylene, ammonia and molecular oxygen in thepresence of a catalyst, contacting the reacted gas with liquidcontaining water and an acid in a multi-stage quenching tower havingthree or more chambers and supplying the liquid contacted with thereacted gas in the second quenching chamber to the first quenchingchamber.

The quenching tower used in the present invention is a multi-stagequenching tower divided into three or more levels and each levelconsists of one chamber. One example of the tower of the presentinvention is shown in FIG. 1. If necessary, each chamber can be furtherdivided into more than two levels. Packed beds for removing impuritiesand spray nozzles for supplying a liquid can be arranged in thequenching tower.

Hereinafter, an example of the production method of the presentinvention will be described referring to the multi-stage quenching towerof FIG. 1. The multi-stage quenching tower of FIG. 1 is divided intothree levels. Namely, it comprises first, second and third quenchingchambers (2), (3) and (4), respectively. The tower has a diameter of 2.5to 3.0 m and a height of 12 to 14 m. The second and third quenchingchambers have packed beds (21) and (22) packed with porcelain Raschigrings and spray nozzles on the end part of liquid supply pipes (7), (10)and (13), respectively.

The reacted gas obtained by reacting propylene, ammonia and molecularoxygen is first supplied to the first quenching chamber (2) through agas leading pipe (1). In general, the composition of the reacted gas isas follows:

    ______________________________________                                        <Component>          <vol. %>                                                 ______________________________________                                        acrylonitrile        6.0-7.0                                                  ammonia              0.0-1.0                                                  propylene and propane                                                                              0.2-0.6                                                  acetonitrile         0.1-0.5                                                  hydrocyanic acid     0.8-1.6                                                  non-condensable gas  60.0-67.0                                                water vapor          25.0-30.0                                                other materials (acrolein,                                                                         0.0-0.2                                                  high boiling point compounds, etc.)                                           ______________________________________                                    

This reacted gas is supplied to the first quenching chamber (2) at about15 to 17 T/Hr and 230° to 280° C. "T" is defined as a measure of massand 1T corresponds to 1000 kg. The reacted gas is cooled and washed bycontacting it with the liquid containing water and an acid such assulfuric acid. The liquid is supplied to the first quenching chamber (2)through a liquid supply pipe (7) against the flow of the gas at 150 to170 T/Hr. The temperature of the liquid is preferably between 85° and95° C. The pH value of the liquid is preferably between about 5 and 6.The pH value can be adjusted by adding an acid to the liquid through anacid supply pipe (19). If necessary, the liquid may be cooled by acooler (6).

Most of unreacted ammonia, high boiling point compounds, polymers,scattered catalysts and the like are removed in the first quenchingchamber (2). The term "high boiling point compounds" means anitrogen-containing hydrocarbon polymer produced in the reactor,polymers obtained by polymerizing acrolein, acrylonitrile or hydrocyanicacid, and polymers obtained by polymerizing β-aminopropionitrileobtained by reacting acrylonitrile and ammonia or cyanohydrin obtainedby reacting acrolein and hydrocyanic acid.

The temperature of the liquid discharged through a discharging pipe (5)at the bottom of the quenching tower at the time when the inside of thefirst quenching chamber reaches its equilibrium state is 80° to 100° C.though it changes to some degree depending on the temperature andcomposition of the reacted gas supplied to the chamber. At that time,the liquid in the first quenching chamber (2) becomes a solutioncontaining an extremely small amount of acrylonitrile, ammonium sulfateand viscous high boiling point compounds and so on. The liquid isdischarged from the first quenching chamber (2) out of the tower througha liquid discharging pipe (14) at about 0.5 to 1.0 T/Hr. The pressure atthe bottom of the tower is about 0.4 to 0.6 kg/cm².g.

The reacted gas treated in the first quenching chamber (2) istransferred to the second quenching chamber (3), and the removal ofimpurities left in the reacted gas is continued. The gas transferredfrom the first quenching chamber (2) to the second quenching chamber (3)contains unreacted ammonia, high boiling point compounds, polymers, ascattered catalyst and the like which are not removed in the firstquenching chamber (2). A liquid containing high boiling point compounds,polymers, scattered catalysts and ammonium sulfate is entrained with thegas in the form of mist.

In the second quenching chamber (3), the reacted gas may be treatedaccording to the same method as the first quenching chamber (2) or bythe conventional methods. For example, the liquid supplied to the secondquenching chamber (3) is cooled down to 68° to 73° C. by a cooler (9)and supplied into the chamber through a liquid supply pipe (10) at 150to 170 T/Hr. If necessary, an acid can be supplied through an acidsupply pipe (20).

In this chamber, the above-mentioned impurities remaining in the reactedgas are removed, and the water vapor initially contained in the reactedgas is partially liquefied by the condensation. Therefore, the amount ofliquid increased thereby is discharged from this chamber (3).

In the present invention, the liquid discharged from the secondquenching chamber (3) through a liquid discharging pipe (8) is suppliedinto the first quenching chamber (2) through a liquid supply pipe (15).In addition, the discharged liquid also can be supplied to anacrylonitrile recovery process through a liquid discharging pipe (16).The amount of liquid supplied from the second quenching chamber (3) intothe first quenching chamber (2) is preferably 10 to 60 wt. %, morepreferably 30 to 50 wt. %, of the total amount of the liquid suppliedfrom the second quenching chamber (3) to the first quenching chamber (2)and the liquid supplied from quenching chambers above second quenchingchamber (3) to an acrylonitrile recovery process, which corresponds tothe total amount of the liquid supplied from the water discharging pipes(16) and (17).

The temperature of the liquid discharged from the second quenchingchamber (3) at the time when the inside of the second quenching chamber(3) reaches its equilibrium state is between 70° and 90° C. though itchanges to some degree depending on the temperature, composition of thereacted gas transferred in the chamber and the load of the cooler (9).The liquid is discharged from the chamber at about 0.5 to 2.7 T/Hr. Thepressure at the bottom of the second quenching chamber is about 0.3 to0.6 kg/cm².g.

The gas treated in the second quenching chamber (3) is transferred tothe third quenching chamber (4). The transferred gas contains only avery small quantity of the impurities mentioned above. In the thirdquenching chamber (4), the reacted gas also can be treated according tothe same method as the first quenching chamber or by conventionalmethods. For example, the liquid is discharged through a liquiddischarging pipe (11) and cooled down to about 36° to 38° C. by a cooler(12), transferred from spray nozzles through a liquid supply pipe (13),and supplied to the chamber at 60 to 80 T/Hr. If necessary, an acid canbe added to the liquid again.

The temperature of the discharged liquid at the time when the inside ofthe third quenching chamber reaches its equilibrium state is between 60°to 80° C. though it changes to some degree depending on the temperature,composition of the reacted gas transferred in the chamber and the loadof the cooler (12). The liquid is discharged from the third quenchingchamber (4) at about 2 to 4 T/Hr. The pressure at the bottom of thethird quenching chamber (4) is about 0.3 to 0.5 kg/cm².g.

The amount of the water vapor to be condensed in each quenching chambercan be adjusted by changing a load of coolers to control the temperatureof the gas transferred to the following chamber or process.

The reacted gas, from which high boiling point compounds and the likeare removed in a multi-stage quenching tower, is transferred to aprocess for recovering acrylonitrile. The liquid discharged from thethird quenching chamber (4) out of the tower through a liquiddischarging pipe (17) can be recycled after recovering acrylonitrilecontained in the liquid.

Hereinafter, the process for recovering acrylonitrile will be described.The gas contains acrylonitrile, acetonitrile, hydrocyanic acid, carbondioxide, carbon monoxide, nitrogen, unreacted propane and unreactedpropylene and so on, and the temperature of the reacted gas passingthrough a gas transferring port (18) is between about 37° and 39° C. Thegas is transferred to an absorption column to absorb acrylonitrile,hydrocyanic acid, acetonitrile and the like into water. The gas which isnot absorbed in the absorption column is incinerated or emitted in theair, if necessary, after purification.

The liquid discharged from the bottom of the absorption column istransferred to the acrylonitrile recovery column together with theliquid discharged through liquid discharging pipes (17) and (16).

In the recovery column, acetonitrile is removed by extractivedistillation using water as an extractant. From the top of the column,the liquid containing acrylonitrile, hydrocyanic acid and a small amountof water is discharged and transferred to the purification process foracrylonitrile. The liquid discharged from the bottom of the column istransferred to a stripper to remove acetonitrile.

The acrylonitrile recovery column and the stripper are both equippedwith reboilers. The liquid discharged from the bottom of the tower issupplied to the reboilers. Further, steam is supplied to the reboilerswhile the liquid is circulated to remove acrylonitrile, hydrocyanicacid, acetonitrile and the like.

According to the present invention, the high boiling point compounds areefficiently removed and clogging is prevented in an acrylonitrilerecovery process so that the productivity of acrylonitrile is enhanced.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic view of the multi-stage quenching tower used inthe present invention.

Description of Numerals

1: gas lead in pipe

2: first quenching chamber

3: second quenching chamber

4: third quenching chamber

5: liquid discharging pipe

6: cooler

7: liquid supply pipe

8: liquid discharging pipe

9: cooler

10: liquid supply pipe

11: liquid discharging pipe

12: cooler

13: liquid supply pipe

14: liquid discharging pipe out of the tower

15: liquid supply pipe

16: liquid discharging pipe oui,. of the tower

17: liquid discharging pipe out of the tower

18: gas transferring port

19: acid supply pipe

20: acid supply pipe

21: packed bed

22: packed bed

BEST MODE FOR CARRYING OUT THE INVENTION

Measurement of concentration!

In each Example, the concentrations of high boiling point compounds andammonium sulfate remaining in the liquid after contacting reacted gasand the liquid were measured according to the following method.

The sample liquid taken out from each quenching chamber was evaporatedunder a warm bath of 90° C., dried at 105° C. and allowed to cool in adesiccator. The residue was dissolved into warm water at 40° C., and theamount of ammonium sulfate was measured by a formaldehyde method. Theformaldehyde method is a measuring method comprising adding formaldehydeto ammonium sulfate, and titrating the generated sulfuric acid byalkali.

The concentrations of high boiling point compounds and ammonium sulfatewere calculated by applying the amount of the sample and the amount ofthe residue (the total amount of the dried high boiling point compoundsand ammonium sulfate) to the following formula. The amount of the highboiling point compounds was calculated by subtracting the amount ofammonium sulfate from the amount of the above mentioned residue.

    C(wt. %)=(T/S)×100

C: concentration of the total amount of high boiling point compounds andan ammonium sulfate

T: total amount of dried high boiling point compounds and dried ammoniumsulfate

S: amount of the sample obtained

Further, the concentration of acrylonitrile was measured by a gaschromatography.

<EXAMPLE 1>

An experiment was carried out using the multi-stage quenching towerwhose inside was divided into three levels as disclosed in FIG. 1. Thequenching tower had a diameter of 2.7 m and a height of 12.7 m. In eachquenching chamber of the tower, spray nozzles were arranged on the endpart of liquid supply pipes. The packed beds (21) and (22) of the secondquenching chamber (3) and the third quenching chamber (4) were filledwith porcelain Raschig rings.

Propylene, ammonia and air were reacted in a reaction tower to obtain areacted gas with the composition as follows:

    ______________________________________                                        <Component>        <vol. %>                                                   ______________________________________                                        acrylonitrile      6.5                                                        ammonia            0.5                                                        propylene and propane                                                                            0.4                                                        acetonitrile       0.3                                                        hydrocyanic acid   1.2                                                        non-condensed gas  63.5                                                       water vapor        27.5                                                       other materials (acrolein &                                                                      0.1                                                        high boiling point compound)                                                  ______________________________________                                    

The reacted gas was introduced through the gas lead in pipe (1) into thefirst quenching chamber (2) at 15 T/Hr. The temperature of the gas was250° C.

The liquid discharged from the bottom of the first quenching chamber (2)and the second quenching chamber (3) at 90° C., and containing sulfuricacid which was added to adjust the pH value of the liquid to 5.3, wassupplied through a liquid supply pipe (7) to the first quenching chamber(2) at 160 T/Hr. The liquid contacted with the gas was circulatedthrough the liquid discharging pipe (5) and the liquid supply pipe (7)and supplied to the quenching chamber. The liquid was discharged fromthe first quenching chamber (2) to the outside of the tower through theliquid discharging pipe (14) at 0.8 T/Hr.

The reacted gas treated in the first quenching chamber (2) wastransferred to the second quenching chamber (3). The liquid of 70° C.,whose pH value was adjusted to 5.5 with sulfuric acid, was supplied at160 T/Hr. The liquid contacted with the gas was circulated and suppliedin the same way as in the first quenching chamber (2). The liquid wasdischarged from the second quenching chamber (3) at 1.8 T/Hr andsupplied to the first quenching chamber (2) through a liquid supply pipe(15).

The reacted gas treated in the second quenching chamber (3) wastransferred to the third quenching chamber (4). The liquid of 37° C.,whose pH value was 5.5, was supplied at 70 T/Hr. The liquid contactedwith the gas was circulated and supplied in the same way as in the firstquenching chamber. The liquid was discharged from the third quenchingchamber (4) at 2.7 T/Hr and sent to the acrylonitrile recovery process.

The concentrations of high boiling point compounds, ammonium sulfate andacrylonitrile, which remain in the liquid discharged from each chamberat the time when the inside of the multi-stage quenching tower reachedits equilibrium state, are shown in Table 1.

In the acrylonitrile recovery process, the reboilers arranged in theacrylonitrile recovery column and the stripper were operated withoutcleaning their insides for a year. When the insides of the column andthe stripper were inspected after one-year operation, no polymer and thelike were observed.

<COMPARATIVE EXAMPLE 1>

The reacted gas with the composition shown in Example 1 was treated inthe same manner as in Example 1 except that the liquid was supplied tothe acrylonitrile recovery process without supplying the liquid to thefirst quenching chamber (2).

The reboilers in the acrylonitrile recovery column and the stripperclogged after 4.5 month and 3 month operation, respectively. Thereboilers needed cleaning.

Industrial Application

According to the method of the present invention, problems such asclogging caused by high boiling point compounds or polymers can beavoided in an acrylonitrile recovery process. As a result, theproductivity of acrylonitrile can be increased.

                  TABLE 1                                                         ______________________________________                                                1st quenching                                                                           2nd quenching                                                                            3rd quenching                                            chamber   chamber    chamber                                          ______________________________________                                        high boiling                                                                            18.4        3.5        0.2                                          point compounds                                                               (wt. %)                                                                       ammonium  28.0        0.8        0.01                                         sulfate                                                                       (wt. %)                                                                       acrylonitrile                                                                           0.08        0.8        5.2                                          (wt. %)                                                                       ______________________________________                                    

What is claimed is:
 1. A method for producing acrylonitrile in thepresence of a catalyst comprising: reacting ammonia, molecular oxygen,and at least one reactant chosen from the group consisting of propyleneand propane to form a reacted gas; contacting the reacted gas with aliquid containing water and an acid in a multi-stage quenching towercomprising at least three quenching chambers, a first quenching chamberlocated at the base of the quenching tower, a second quenching chamberlocated immediately above the first quenching chamber and a thirdquenching chamber located immediately above the second quenchingchamber; and transferring the liquid in contact with the reacted gas inthe second quenching chamber to the first quenching chamber.
 2. Themethod for producing acrylonitrile according to claim 1, wherein anamount of the liquid in contact with the reacted gas in the secondquenching chamber transferred to the first quenching chamber is 10 wt. %to 60 wt. % of the amount of the liquid supplied from liquid dischargingpipes discharging liquid from the quenching chambers above the firstquenching chamber plus the amount of liquid supplied from the secondquenching chamber to the first quenching chamber.